Day 391

Last week in Brewhouse Calculations we calculated how many yeast cells per millilitre we wanted munching on our wort sugar, based on the gravity of the wort. Today, we continued with calculating how many total yeast cells our wort would need (obviously dependent on how much wort there is); and then how much yeast slurry we would add. So including last week’s calculations, it’s a simple three-step process:

  1. Decide on a pitching rate (how many yeast cells per millilitre) and multiply that by the gravity of the wort.
  2. Calculate  how many yeast cells you need in total by multiplying the number of cells per millilitre from Step 1 by the total volume of your wort, measured in millilitres.
  3. Calculate the volume of your yeast slurry that contains that many yeast cells.

So if we have 7.5 hectolitres of wort (which equals 750 litres, which equals 750,000 mL, which can also be expressed as 7.5 x 105 mL) that has a gravity of 18°P; and a yeast slurry that has 1.0 x 108 yeast cells per mL; and we want a pitching rate of 1.3 x 106 yeast cells per mL per °P, then:

  1. Pitching rate = basic rate x gravity = 1.3 x 106 x 18°P = 2.34 x 107 yeast cells/mL
  2. Total number of yeast cells needed = pitching rate x volumemL = 2.34 x 107 cells/mL  x  7.5 x 105 mL  =  1.75  x  1013 cells
  3. Volume of slurry needed = total cells needed ÷ slurry’s cells/mL = 1.75  x  1013 cells ÷ 1.0 x 108 cells/mL = 1.75 x 105 mL = 175 L

That’s a lot of slurry.

That was actually some review from last year, so we were able to quickly move on to brew water calculations. First up was calculations involving calcium. This funny little atom is needed at every step of the brewing process, from the mash right through to fermentation, so brewers often add more calcium in the form of calcium sulphate (aka gypsum) or calcium chloride (which can take on various forms, depending on how many water molecules are attached to it: the dihydrate form has 2 water molecules attached, the hexahydrate form has six.) The problem when doing these calculations, is that calcium only makes up part of the molecule: only 23% of calcium sulphate is actually calcium, for instance, and only 18% of a molecule of calcium chloride hexahydrate is calcium. So when we want to bring the calcium content of our water up to a certain concentration, it is another three-step process:

  1. Subtract the actual calcium content of the water (in mg/L) from the desired concentration to get the needed concentration to be added.
  2. Divide the needed concentration by the actual percentage of calcium in the compound to get the amount of the compound (in mg/L) to be added.
  3. Multiply this by the volume of the water to arrive at the amount of compound to be added.

So, if we have 75.7 L of mash water with an existing calcium content of 7 mg/L, and we want to raise that to 100 mg/L using calcium chloride dihydrate, which has a calcium content of 27%:

  1. concentration needed = 100 mg/L – 7 mg/L = 93 mg/L
  2. concentration of CaCl2·2H2O needed = 93 mg/L ÷ 27% = 344.4 mg/L
  3. total CaCl2·2H2O needed = 344.4 mg/L x 75.7 L = 2.6 x 103 mg = 26 grams

All of these equations and some problems are listed in our text book (A Handbook of Basic Brewing Calculations by Stephen Holle). I have a copy, but some of the other second-year students have run into a curious problem: This text arrived in the college bookstore a a couple of weeks after the semester had started, apparently just as a first year Brewmaster student was in the store. He bought one, and as I understand it, the rest of his classmates saw it and rushed to the store to buy up all the remaining copies. The bookstore is now refusing to order any more copies for second-year students who actually need the text book, since, hey, the bookstore originally ordered the correct amount for the year. Now not only will most second-year students have to order a copy from the States and hope it arrives pronto, but next year the bookstore will order enough copies for the new second-year students–and the new text books will sit on the shelf, because all the new second-year students already have one.

There has got to be a better system.

On to History of Beer, where we learned about the gradual changeover from monastery brewing to increased private commercialization.

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